Sound recording apparatus



April 21, 1942. E-L' BELAR SOUND RECORDING APPARATUS Filed March 30,1940 2 Sheets-Shet l Bnventor He rbe rt B e la attorn eg April 21, 1942.H, BELAR 2,280,740

SOUND RECORDING APPARATUS Filec} March so, 1940 2 Sheet sSheet 2 P we ISuventor Hei'be rt Be Z ar ttomeg Patented Apr. 21, 1 942 sown RECORDINGAPPARATUS Herbert Belar, Philadelphia, Y Pa., assignor to RadioCorporation of America, a corporation of Delaware Application Marcli30,1940, Serial No. 326,836

7 Claims.

This invention relates to sound recording apparatus, and-moreparticularly to a ground noise reduction amplifier for use in suchrecording apparatus and the like.

It is known that thetransparent unmodulated areas on sound film tend toproduce noise during the reproduction of sound due to extraneousmaterials which may adhere to the surface of the clear portion of thefilm and due to developed silver grains in the emulsion. The reductionof such noise, by rendering the unmodulated portion of the backgroundor. positive film opaque, is'

described and claimed, for example, in McDowell Reissue Patent No.21,389. The said patent discloses the use of a rectifying amplifier intowhich a portion of the audio frequency current is fed. This amplifier,commonly referred toss a ground noise reduction amplifier, rectifies andfilters the that the background in the positive will be 4 printedopaque.

There have heretofore been difficulties in the construction ofsuchamplifiers due to the fact that the filtering circuit which is capableof 5 filtering out the low audio frequencies necessarily produces acorrespoiiding time delay. This time delay causes, of course, acorresponding delay in the response of the shutter in followingthe-envelope of the sound waves and causes the l clipping of the peakswhen there.is a sudden increase in the amplitude of the sound.

With early equipment, the frequency range was i not very great and thisclipping did not become serious. However, improvements in the soundrecording and film moving equipment have permitted the frequency rangeof recording to be greatly extended so that the recording of frequenciesas low as 50 or even 30 cycles has become customary. The recording ofsuch low frequencies has necessitated the improvement of the filteringin the ground noise reduction amplifier,

while, at the same time,-the improved quality of 4 the soundreproduction has caused the audiences to become more critical of anydefects in the.

ground noise reduction. Improving the speed of operation of the groundnoise reduction apparatus requires the use of less filtering, which iscompletely inconsistent with the requirement for better filtering duetothe increased frequency range in such a circuit as that disclosed byMo- Dowell, for example. p

The apparatus of the present application accomplishes either a speedingup of the operation of the ground noise reduction'apparatus, or animprovement of the filtering produced, or both. This result isaccomplished by passing the audio frequency input current to the groundnoise reduction amplifier through a series of frequency doublersconsisting of full wave rectifiers and transformers so that the lowestfrequency to be filtered is several times the lowest frequency which isrecorded, thereby permitting use of a smaller filter withcorrespondingly less time delay to secure the same degree of filtering.

One object 'of the invention is to provide an improved ground noisereduction amplifier.

Another objectof the invention is to provide an apparatus which willderive a direct current corresponding to the envelope of an audiofrequency current with maximum filtering and at minimum time delay. 7

Another object of the invention is to provide an amplifier including oneor more frequency doublers for increasing the minimum frequency fed to afilter circuit, thereby improving the efficacy of the filter circuit.

Another object of the invention is to provide the sound recordingapparatuswhich will produce a record having better ground noisereduction and higher fidelity of recording than has heretofore beenproduced.

Other and incidental objects of the invention will be apparent to thoseskilled in the art from a reading of the following specification and aninspection of the accompanying drawings, in which Figure 1 is aschematic diagram of one form of the invention in which the directcurrent out put increases on an increase in the audio or A. C.

Figure 2 is a schematic diagram of a second 1 III, the secondary ofwhichis provided with a potentiometer 20 or other equivalent device forcontrolling the input to the first amplifier tube ll. Grid bias .ior thefirst amplifier tube may be provided by the usual cathode resistor 12bypassed by the usual cathode condenser 3. The output from the tube H isfed through the transformer I 4 to the rectifiers i5 and 16, whichconstitute a full wave rectifier and may be in a common envelope, as,for example, in the type of tube known as the 61-16. The output from therectifiers l5 and I6 is fed to the primary of the transformer l? which,like the transformer I4, has a push-pull secondary. It will be apparentthat each half wave passing through the transformer It will produce aseparate impulse in the same direction through the primary of thetransformer ll, thereby inducing in the secondary of the transformer i'lalternating currents of twice the frequency of those in the trans formerl4. Theoutput from the transformer ill is fed to a second full waverectifier I8 and I9 which again may be a type 6H6 tube; It will beapparent that each half impulse from the transformer l1 produces aseparate impulse through the rectifiers in the same direction, therebyagain doubling the frequency. The output from this second pair ofrectifiers is-fed into the filter circuit including the capacitors Z6,22 and 23', and the resistorsifl, 25 and 28. It will be apparent that ifa frequency of, say, 50 cycles is applied to the transformer ill, theinput to the filtercircuit due to the two doublings will be a pulsatingdirect current with a pulsation frequency of 200 cycles, which can besmoothed out to its average value by a filter circuit having a muchfaster time constant than would be the case for the original 50 cycles,if rectified.

In the form of the invention shown in Fig. 1,.

the grid lead of the amplifier 21 is connected to the cathodes of therectifiers l8 and i9, and therefore the output of the rectifiers willcause ent recording it is often desired to record a fre-- quency rangefrom 30 to 10,000 cyclesand the the range of output frequencies wouldbecome the grid to become more positive, thereby increasing the platecurrent on an increase of A.-C. input. 'An appropriate meter 28. may beinserted in theplate. circuit of the tube 2'1 to indicate the D.-C.output and thereby indicatethe operation of the ground noise reductionshutter which later may be constructed, for example, in accordance withBaker Patent No. 2,102,776.

In the form of the invention shown inF-ig. 2, the cathodes of therectifiers I8 and iii are connected to the cathode of the D.C. amplifiertube 35 through a potentiometer 34, while the center tap of thetransformer I lie connected through the filter circuit to the grid ofthe tube 35. This filter circuit is quite similar to that shown in Fig.1 consisting of the capacitors 30 and 3] and the resistors 32, 33 and.Inthis circuit, due to the reversal of connections, an increase in audioare necessary to secure the required speed of operation by decreasingthe time constant of the filter circuit, v

In the form of the invention shown in Fig.3,

a multiplicity of frequency-doubling stages are used in -thiscase five,which gives a correspondingly'high speed of response. However, the useof such a number of doubling stages by themselves is impracticalparticularly where a wide frequency range is to be handled. For example,in pres- 960 to 320,000 cycles, It will be apparent that the higherfrequencies would not be transmitted by the audio frequency transformersused and a separate circuit is therefore used to transmit the higherfrequencies.

In the operation of this circuit, the audio fre quency input passesthrough the transformer ID, the potentiometer 20, the amplifier tube IIand the transformer M, as in the other forms of the invention. Insteadof the single interstage transformer ll of Figs. 1 and 2, there areprovided, however, four such transformers, Ha, H2), H0 and lid. In thepresent case, the full wave rectifier tubes are indicated in the mannercon ventional for the type 6H6 tubes at 3B, 31, 38, 39 and 40 and itwill be apparent that the output of the tube 40 will-havebeen doubled infrequency a corresponding number of times, as pointed out above.

A portion of the output from the transformer I0 is also applied to theamplifier tube 53 through the high pass filter including resistors 50'and 5i and the capacitor 52. This high, pass filter is designedaccording to the characteristics of the circuit from the transformer Hto the tube 40 so that the frequencies of the input to the tube 53 willcorrespond to those frequencies which have been attenuated in orrejected by the doubier circuits. The output of the tube 53 passesthrough thetransformer 56 to the rectifier 5,1 which, in this case, isshown used as a halfwave rectifier so as to avoid any frequencydoudoubling. The output from the rectifier tube 40 is passed through thefilter circuit including the elements pointed out above and also thecapacitor 5! and the resistor 24 in addition to the resistors 59 and B0and the capacitor 5!. Appropriate grid bias may be applied to the tube21 through the potentiometer 53cwhich is connected to one side of theplate supply and the resistor 62 which is connected to the other side ofthe plate supply. It will be apparent that in thisform of the inventionall of the lower frequencies will be doubled five times leaving thelowest frequency in the output of the tube 40 something of the order of900 to 1000 cycles with the other frequencies increasingcorrespondingly. The frequencies selectedby the filter circuit in theupper portion of this circuit and the degreeof amplification securedthrough the amplifier 53 are so chosen that they exactly offset the highfrequency losses through the doubler circuits and the output from thetube 21 accordingly corresponds acv curately with the envelope of theaudio impulses introduced through the transformer Hi. This outputfurther is'fiitered to a very high degree but without any appreciabletime lagas' the filter I circuit in the input to the-tube 2'! can bemade 1 frequencies applied thereto.

very small, having no frequencies applied to it less than about 900cycles.

It will be apparent that the modification of the apparatus as shown inFig. 3 can be extended to any desired degree. For example, the firstdoubling amplifier may have five doubling stages, as illustrated in Fig.3, and be fed only with the lower group of frequencies. A seconddoublingamplifier might be fed through a band-pass filter transmitting a groupof frequencies slightly higher than those fed through the first doublerand the \second doubler having a smaller number of doubling stages, asfor example, three. Still a third doubling'amplifier may be providedhaving, for example, only a single doubling stage and likewise fedthrough a band-pass filter transmitting a .higher group of frequenciesand the final rectifying amplifier of the group would be small to thatshown in the upper portion of Fig. 3 but fed with only the highest audiofrequencies which require no doubling.

All the outputs of the rectifying amplifiers in .such an arrangementwould be combined to provide the desired envelope current preferablybeing combined through a single filter circuit as shown in Fig.3although separate filter circuits could be provided if desired and theoutputs thereafter combined in the D.-C. amplifier. The number ofdoubling amplifiers to be used, the

number of doubling stages in each doubling amplifier and the provisionof a rectifying amplifier without a doubler must, of course, be chosenwith proper regard to the circumstances under which the circuit is tooperate. It will be apparent, for

example, that an apparatus using a large-num-- and at least onefrequency doubler preceding said filter, said filter circuit having suchcharacteristics that it will not properly filter A.-C. input frequenciesof the order of 50 cycles but will adequately filter the doubledfrequencies applied thereto. 4

2. An amplifier of the type described having an A.-C. input and a D.-C.output corresponding to the envelope of the A.C. input, said amplifierincluding a filter circuit and at least one full wave rectifier servingas a frequency doubler preceding said filter circuit, said filtercircuit having such characteristics that it will not properly filterA.-C. input frequencies of the order of 30 cycles but will adequatelyfilter the doubled 3. An amplifier of the type described having an A.-C.input and a D.-C. output corresponding to the envelope of the A.-C.input, said amplifier including a filter circuit and a plurality of fullwave rectifiers serving as frequency doublers preceding said" filtercircuit, said filter circuit having such characteristics that it willnot properly filter A.-C. input frequencies of the order of 30 cyclesbut will adequately filter the doubled frequencies applied thereto.

4. An amplifier of the type described having an A.-'C. input and a D.-C.output corresponding to the envelope of the A.-C. input, said amplifierincluding a first amplifying stage adapted to amplify the A.-C. input, aplurality of full wave rectifier frequency doubler stages, a filtercircuit, and a D.-C. amplifier stage actuated from the output of saidfilter circuit, said filter circuit having such characteristics that itwill not properly filter A.-C. input frequencies of the order of 30cycles but will adequately filter the doubled frequencies appliedthereto.

5. Apparatus of the class described including an amplifier having anA.-C. input and a D.-C. output corresponding to the envelope of theA.-C. input, said amplifier including at least one frequency doubler,rectifying means and output filter means, said apparatus including asecond amplifier connected to the same A.-C. input, having a D.-C.output corresponding to the envelope of the A.-C. input and includingrectifying means, the output of said second amplifier being connected tothe same output filter means as the first amplifier. l

6. Apparatus of the class described including an amplifier having anA.-C. input and a D.-C. output corresponding to the envelope of theA.-C. input, said amplifier including at least one frequency doubler,rectifying means and output including at least one amplifying stage, afilter,

filter means, said apparatus including a second amplifier connected tothe same A.-C. input, having a D.-C. output corresponding to theenvelope of the A.-C. input and including rectifying means, the outputof said second amplifier being connected to the same output filter meansas the first amplifier, and high-pass filter means connected between thesaid A.-C. input and said second amplifier.

to the envelope of the A.-C. input and includin rectifying means and aless number of doublers than the first amplifier, the output of saidadditional amplifier being connected to the same output filter meansasthe first amplifier.

HERBERT BELAR.

